OBJECTIVE: To evaluate contributions to parental effort by mothers, fathers, and older siblings, and to identify potential endocrine and experiential determinant of variability in these patterns. RESULTS Through the use of the Primate Center core assay services and facilities, we were able to complete an initial validation of an assay for marmoset urinary prolactin, suggesting that the assay may be useful for helping us assess the role of this pituitary peptide in regulating parental care. Further, we have shown that group demography affects patterns of parental care in intriguing ways. The presence of sibling "helpers" reduces effort by the father, but has not effect on levels of maternal effort. FUTURE DIRECTIONS We will assess the degree to which variation in parental care by various group members is associated with individual differences in levels of pituitary peptide and sex steroid hormone concentrations. KEY WORDS Prolactin, estrogen, progesterone, urinary hormones